The relationship of Western Disturbances to precipitation in the upper Indus River basin

The upper Indus River basin is characterised by heavy precipitation falling near the foothills of the major mountain ranges, during two wet seasons: winter and summer. Winter precipitation is known to be related to the passing of upper-level synoptic systems embedded in the subtropical westerly jet called Western Disturbances. Here, we investigate the precipitation variability in relation to the Western Disturbances at the synoptic scale, using ERA5 reanalysis data. We take advantage of the results of a previous study that showed that the precipitation is mostly triggered by the forced uplift of a low-level moisture-rich southerly flow across the ranges. We show that the low-level southerly wind triggering the precipitation is produced by the interaction of a Western Disturbance with a baroclinic front located between the Iranian plateau and the Arabian Sea. Ahead of the Western Disturbance, low-level winds draw moisture from the extreme north of the Arabian Sea, the Persian Gulf, and to a lower extent, the Red Sea. At the rear, moisture is depleted by the advection of continental dry air in the Indus River basin. However, the balance between moisture drawing and depletion depends on the characteristics of the Western Disturbance, leading to differences in precipitation intensity. We found the jet position and western Russia blockings to play a role in this. These findings offer clues to understand the longer-term precipitation variability in the area.